1. Field of the Invention
The present invention is in the field of water treatment and particularly relates to a system for determining the effectiveness of a water softener or a similar water treating device.
2. The Prior Art
A water softener of the ion exchange type is typical of the type of apparatus the present invention is used to monitor. Such a water softener includes a chamber through which the water to be softened flows. The chamber contains a bed of pellets of an ion exchange resin. As the incoming hard water passes through the bed, the magnesium and calcium ions are exchanged for sodium ions which pass into the water, softening it.
With extended use the ion exchange bed becomes depleted of sodium ions, and the efficiency of the softener decreases. When the efficiency has decreased to the point where the water is not soft enough for the intended use, the ion exchange bed must be regenerated.
As the ion bed gradually becomes depleted and the treated water becomes less soft, the electrical conductivity of the treated water gradually decreases until ultimately it approaches the conductivity of the incoming untreated water.
The system of the present invention continually senses the conductivity of the untreated water and the conductivity of the treated water. When a freshly charged ion exchange bed is first put into use, the difference in the conductivities is greatest and is therefore designated as 100% efficiency. With use, the difference decreases and ultimately approaches zero.
Two major families of conductivity sensors have been identified in the prior art: the direct contact type of instrument and the inductance loop type of instrument. The system of the present invention employs a modified form of the inductance loop instrument.
U.S. Pat. No. 3,246,759, issued Apr. 19, 1960 to Matalon, describes a direct contact type of instrument in which electrodes are positioned in the resin bed and connected to a detection circuit. Current is impressed on the circuit and the amplitude of the current is responsive to the conductivity of the resin-water system between the electrodes. As the sodium ions of the resin are replaced by magnesium or calcium ions from the water, the conductivity of the resin-water system decreases and the current flow decreases accordingly. A major deficiency of direct contact instruments of this type is that the electrodes become contaminated over a period of time by the ions present in the water, thereby making accurate measurements over a period of time impossible. As mentioned above, the present invention employs a modified form of inductance-loop instrument.
In a typical inductance loop type of instrument, an exciting toroid and a pick-up toroid are juxtaposed in the water. An alternating voltage is applied to the exciting toroid, and this causes a weaker alternating voltage to be induced in the pick-up toroid because of the coupling provided between the toroids by a current loop in the water linking the toroids.
As will be discussed below, this type of instrument is susceptible to certain types of deficiencies which the system of the present invention overcomes.
In a technical paper entitled "A Probe Type Induction Conductivity Cell" by E. E. Aagaard and R. H. vanHaagen of Oceanic Instruments, Inc., Houghton, Wash., published around 1962, there are described circuits in which a transformer bridge couples a pair of toroids which are also coupled by the unknown conductivity of the liquid. The purpose of the transformer bridge is to enable a nulling type of operation that is implemented by a potentiometer in the bridge circuit. A capacitor is connected across the pick-up toroid to provide a tuned circuit which is more sensitive to the signal being nulled and which tends to filter out noise and to reduce null signal phase shift with respect to the exciting voltage. This is shown in FIG. 3 of the reference. In connection with FIGS. 5 and 6 of the reference, the authors mention that a small quadrature (reactive) component may be present in the bridge circuit and that this would interfere with obtaining a sharp null. They state that compensation for this reactive component may be made with a small variable capacitor connected across the poteniometer in the bridge circuit. While this expedient does permit the introduction of an opposing reactance, the adjustment is not automatic.
In U.S. Pat. No. 2,542,057 issued Feb. 20, 1951, Relis discusses the problem of spurious voltages being induced in the pick-up toroid. Relis attributes these spurious voltages partly to flux leakage from the exciting toroid intercepting the pick-up toroid and partly to capacitive coupling between the toroids. Relis' solution to this problem is to provide a voltage injecting network to supply a bucking voltage of adjustable phase and magnitude to nullify the spurious voltages. This solution requires that the system be calibrated prior to use by a time-consuming iterative procedure. The need for this procedure is eliminated by the system of the present invention.